In recent years, due to the advancement in image capturing and display techniques, people's requirement for image quality has become higher and higher; and associated applications for the image capturing and display techniques have been more and more widely performed. The camera function almost has been a necessary function in modern mobile devices such as smartphones, tablet computers and so on. Optical focusing for imaging in development and design of a camera is a crucial part. The automatically focusing system of a camera was under much attention extensively since early days. The development of the key techniques in the automatically focusing system always has been one of the emphases in research and development for camera manufacturers. Nonetheless, most focusing systems of image capturing apparatuses often can focus on a single object only. Even though modern digital cameras provide the multi-target auto-focusing function for focusing multiple objects at different distances concurrently, the acquired image cannot present effect of shallow depth-of-field accurately in the interested region for a user.
In general, a lens can concentrate the light from a fixed distance only. Images departing from the fixed distance will blur gradually. Nonetheless, within a certain range, the degree of image blur is imperceptible. This range is so-called the depth of field (DOF), which means the range of clear imaging for human eyes. The DOF of imaging is usually determined by the distance between the object and the camera lens, the focus distance of the camera, and the aperture value of the lens. As the aperture value of the lens is increased (namely, the aperture is shrunk), the DOF of imaging is increased; as the aperture value of the lens is decreased (namely, the aperture is enlarged), the DOF of imaging is decreased. In addition, change of the DOF is also made by different lens positions during photographing.
Normally, no matter professional cameras, such as digital single-lens reflex cameras, or consumer cameras, such as general entry-level digital cameras, the built-in optical focusing systems will only focus objects at a specific distance under a given DOF condition.
The influence of this limitation on the focusing operation for scenes having only one single object is less significant. However, for scenes having a plurality of objects located at different distances, there will be problems of choosing the focusing location for the objects and of the overall image clarity for the optical focusing system. Modern camera systems mostly adopt the so-called intelligent focusing method, by which image analysis tools are used for detecting the interested region in a frame and then the focusing location is set to this region.
Nonetheless, this intelligent focusing method cannot solve the focusing problem for a plurality of scene regions covering different DOFs concurrently. Thereby, the aperture should be adjusted smaller (namely, the aperture value should be increased) manually for obtaining a large focusing region. Consequently, it is not guaranteed that modern cameras can adjust automatically to the most appropriate aperture while photographing a plurality of objects for focusing the respective theme regions in the image concurrently as well as highlighting the subject in the scene by using the shallow-DOF effect. Besides, modern cameras cannot acquire the largest aperture satisfying the above criteria to photograph in order to reduce the exposure time while capturing images. Thereby, given a smaller aperture, the exposure time is extended, which increases the influence of object movement or camera shaking on capturing images.
In order to satisfy simultaneously the requirements of the effect of highlighting the photographing subject using a shallow DOF, focusing multiple objects at different distances, and using the largest aperture for image capturing while complying with the above criteria, the present invention provides an adaptive focusing method for solving the problems described above.